1. Field of the Invention
The present invention relates to a measurement technology with fiber optics applied thereto and more particularly to an on-vehicle fiber-optic cylinder pressure sensor for measuring the pressure within the cylinder of an internal combustion engine for a motor vehicle.
2. Description of the Related Art
Of the cylinder pressure sensors for controlling the internal combustion engine for motor vehicles, that of a type detecting the pressure electrically by means of piezo-electric power has been the major one. This sensor has its advantage of being small in size and simple in structure. However, it also has such a disadvantage that a signal due to a vibration occurring at the time the valve is opened or shut or a vibration caused by some condition of the road surface is superimposed on the cylinder pressure signal. Further, this signal is easily affected by electric noises from the ignition circuit and the like or external electromagnetic noises, and therefore the S/N ratio is deteriorated making it impossible to obtain a cylinder pressure signal with precision.
Under these circumstances, there is a technology of a fiber-optic cylinder pressure sensor that is resistive to noise and easy to mount around the complicated circumference of an internal combustion engine such as disclosed in the gazette of Japanese Patent Laid-open No. sho 60-166739. According to this technology, an optical fiber is disposed around the internal combustion engine and a portion of it is arranged to pass through a case for pressure detection provided at a position of a washer of a bolt. Since the optical fiber is an electrical insulator, it has such advantages that it is not affected by electric noises, free from short-circuiting, and can be passed through a narrow space around the internal combustion engine.
Further, there is such a cylinder pressure sensor having an optical fiber, together with a pressure detecting portion, mounted within an internal combustion engine. There is also an example in which a cylinder pressure detecting and controlling system for a multi-cylinder engine is built up by using such a sensor, i.e., by using an optical fiber mounted, together with a pressure detecting portion, on the interior of the engine gasket. In this example of system structure, it is adapted such that the optical fiber receives, at the pressure receiving portion provided in each cylinder, a bending deformation corresponding to the cylinder pressure. Since the light quantity propagating through the optical fiber decreases due to the loss of light caused by the bending deformation, the cylinder pressure can be quantitatively measured according to the decrease in the light quantity.
The above described fiber-optic sensor of the prior art can be effectively used for measuring the cylinder pressure in each cylinder when installed within an engine room where it is difficult to secure sufficient space for mounting a plurality of sensors.
However, conditions in the engine room are severe, especially thermally (from -40.degree. C. to +130.degree. C.). Therefore, when enabling the sensor to stably operate for a long time is considered, it is preferred that the sensor be subjected to a lightened thermal burden and provided with suppressed light intensity. In practice, light intensity, for example, of 1 .mu.W or so is enough to measure a combustion pressure. However, it is desired that the combustion pressure be precisely measured even if the return light from the optical fiber is varied for some reason or other. There is, for example, such a method wherein the light intensity from a light emitting device is controlled to be constant by keeping constant the light quantity provided to a monitoring photodiode incorporated in the light emitting means.